Earth Dunes - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 114

11

Earth Dunes

11.2 The Terrestrial Atmosphere

11.1 Introduction

Earth's atmosphere comprises mostly (78 %) nitrogen, with
20 % oxygen, just under 1 % of argon, and traces of other
gases (notably a growing amount of carbon dioxide). Water
vapor is present in the lower atmosphere, in the most humid
regions accounting for about 1 % of the gas volume.
The atmosphere at sea level has a column mass of
10,000 kg/m 2 , equivalent to a column of water 10 m tall (or
a column of mercury 76 cm tall), defined as 1 bar (or
1000 mbar). There is enough gas that an appreciable
amount of light at short visible wavelengths is scattered—
strongly scattered blue light is what our eyes perceive as the
color of the sky (this is not necessarily the case on other
planets). Earth's distance from the sun is such that (with a
modest amount of greenhouse warming) the surface tem-
perature is within the range in which water is a liquid. At
this temperature, the 1 bar surface pressure corresponds to
an atmospheric density of about 1.2 kg/m 3 , about 800 times
less than water.
The drop in temperature with altitude (*10 K/km in dry
conditions, *5 K/km in moist ones) means that rising air
can be cooled to the point where it becomes saturated with
water vapor, which condenses into abundant clouds. Clouds
above Earth appear white because the particles that make
them up are large enough to effectively scatter all wave-
lengths of light in the same way, and the substantial
(*30 %) cloud cover on Earth (Fig. 11.1 ) accounts for the
overall high reflectivity of our planet.
Not infrequently, clouds lead to rain, which feeds rivers
and is (along with glacial action) responsible for much of
the generation of sand on Earth. We show in this chapter a
large number of dunes in proximity to water—an interesting
paradox. Warmth, directly or indirectly from solar heating,
restores water vapor back into the atmosphere, driving our
hydrological cycle (which has some striking parallels with
that on Titan). However, the rainfall and evaporation are not
uniformly distributed.

Humans have swarmed across the planet for tens of thou-
sands of years, and oral traditions regarding dunes, how
they move and how one should move across them are
common to many cultures within arid or semi-arid regions.
Descriptions of various sand dunes exist almost since the
development of writing (see Part 5), but this early infor-
mation is necessarily anecdotal in nature.
It was the European empires in the late 19th century that
combined global reach with a scientific literature to record
findings and debate formation processes. Early work on
dunes and deserts is summarized in, e.g., Goudie (1999): the
major players were the French (principally in the Western
Sahara) and the British (principally in Egypt and, to a lesser
extent, in Arabia and elsewhere). In the English language at
least, a prominent early scholar was the geographer
Vaughan Cornish who in the early 1890s began to sys-
tematically study dunes in various regions. Cornish was
followed by others, culminating in Bagnold's desert
explorations which took place principally in the 1930s.
Aerial photography of dunes was accomplished before
1920, but it was only after World War II that wide aerial
surveys were made. However, as we review in Chap. 18 , it
was the view from space that enabled the systematic cata-
loguing of dune morphologies, scales and orientations
worldwide, in the topic edited by McKee (1979).
This chapter starts with a summary of some important
aspects of Earth's atmosphere that enable sand to be moved
by the wind, followed by a review of the main deserts and
dune fields on Earth, observed migration rates for sand
dunes, the influence of plant cover on dune mobility, the
sources and sinks of sand. It ends with a short discussion of
how the study of sand dunes can provide links to broader
climate issues.